Recent progress in sensor technology has allowed low-Earth-orbit (LEO) satellites to shrink significantly in size, disrupting a legacy industry where traditional satellites cost $500 million to $1 billion to build and launch. Major investments are being made to address the new opportunities this provides for data collection, and many companies are launching nanosatellites and/or microsatellites into LEO to capture this opportunity. The rapidly expanding satellite infrastructure is generating vast amounts of data, reaching nearly 20 PB/year in 2014, with no signs that the trend will level off. To bring the data down from LEO requires an average communication rate of 5 Gb/s, continuously, and that demand will continue to grow.
Typically, most satellites download data via space-to-ground radio-frequency (RF) links, communicating directly with fixed ground stations as the satellites fly within range. The current ground station infrastructure has several key limitations that present significant challenges as the satellite industry continues to grow. Satellite-to-ground communications are “line-of-sight,” meaning that ground stations can receive data directly only from satellites that are above the local horizon. The duration of a satellite pass over a ground station depends on the altitude of the satellite and the distance between the ground station and the ground track of the satellite. With satellites in LEO, the maximum pass duration is typically less than ten minutes.
The frequency of passes is strongly dependent on the satellite orbit parameters and the location of the ground station. For example, a satellite in equatorial orbit will pass over an equatorial ground station on each orbit. With a typical orbital period of 90 minutes, that means 16 passes per day. Similarly, a satellite in a polar orbit will pass over a ground station located at the North Pole once per orbit. On the other hand, the satellite in polar orbit will pass over the equatorial ground station between two and four times per day depending on the alignment of the ground track with the location of the ground station. However, the satellite in equatorial orbit will never pass over the polar ground station. Most LEO satellites are in orbits at some inclination between equatorial and polar, and most ground stations are located at latitudes well south of the North Pole. As such, the pass frequency for any given satellite over any given ground location will typically be three to five times per day for ground stations that are not at high latitude (above about 60 degrees) and not at latitudes higher than the orbital inclination of the satellite.
The consequence of limitations on pass duration and frequency is that a satellite in LEO will be within communication range of a given ground station for no more than 10 percent of a day, and typically for less than 2 percent of a day. These constraints on pass duration and pass frequency are driven by orbital dynamics and can be overcome by increasing the number of ground stations or locating the ground stations at very high latitudes. Avoiding downlink constraints requires a large number of geographically-diverse ground stations that are inherently underutilized.
One method of compensating for the limitations on ground contact time is to increase the data transmission rate during available contact time. High data rates in the RF require some combination of high transmitter power and high-gain antennas on the satellite and the ground station. High power transmitters and high-gain antennas on the space segment are constrained by power and mass limitations on the satellite. High-gain antennas on the ground are not mass limited, but tend to be very large (e.g., 10 meters or more in diameter) and require significant capital investment.
As data produced in LEO increases substantially with more satellites launched, downlink infrastructure must grow to meet demand. However, a more fundamental limitation to downlink rates will be encountered in the future, simply due to the overuse of available RF bandwidth in the space environment. Further, adding new RF ground terminals will not help, because the stations will interfere with one another. Similarly, RF bandwidth is constrained on the space side, i.e., when two satellites are relatively close to one another, their RF signals can interfere.
For new satellite companies leveraging advances in satellite costs, capital investment for an extended ground station network is particularly burdensome because the size and cost of the ground network does not scale with the size of the satellites. Ground station costs have not scaled at the same rate as satellite costs, requiring significant further investment to match growth in satellite capacity.
Laser communication has the potential to provide data rates adequate to handle all the data generated on orbit for the foreseeable future. However, current laser communication technology requires placing expensive laser transmitters on each satellite, and further placing operational constraints on the satellite (e.g., pointing, jitter, etc.) that are often beyond the capability of budget satellites. Thus, there is a need for a laser communication system that can support a broad range of satellites at a moderate cost and without putting undue burden on the satellites.
There have been proposals for a distributed constellation of satellites in Earth orbit, called network satellites, that would enhance the utility of client satellites in Earth orbit by providing a high-bandwidth data link to ground. Client satellites include any satellite in Earth orbit that collects data at a high rate, where high can mean that satellite operations are constrained by availability of communications bandwidth, or that satellite operations require one or more dedicated ground stations. The network satellites receive data from the client satellites, and subsequently, transfer the client data to the ground using optical communication. The proposed system also includes several widely-distributed optical ground stations for receiving data from the network satellites.
In the proposed system, the network satellites were envisioned to have high-gain RF receivers that receive data from client satellites. In addition, the network satellites were envisioned to have laser communication transmitters to send data to the ground. Another form of the network satellites had both laser transmitters and optical receivers (telescopes) to receive data transmitted by other laser systems.
In both types of network satellites, simultaneous operation of both the receive mode and the transmit mode would not be possible because the pointing requirements of the receiver (whether optical or RF) would be incompatible with the pointing requirements of the transmitter. In the current state of the art, this problem is solved using a two-axis gimbal system that allows the laser to point in the required direction and with its required degree of precision, while the rest of the satellite would point as necessary to receive the incoming signal. However, these two-axis gimbal systems tend to be both expensive, and too large for most small satellites.
Thus, an alternative communications relay satellite system may be beneficial.